Anisotropic etching in silicon requires gas mixtures which are able to cause an "in-situ" passivation of the side walls of the structure to be etched. The process parameters are adjusted so that, e.g., carbon containing polymers will form on the sidewall of the structure (of. P. H. Singer, "Today's Plasma Etch Chemistries", Semiconductor International, vol. 11, no. 4, p. 68 (1988)).
For the manufacture of the so called "deep capacitor trenches", Y. T. Lii et al., Electrochem. Soc. Proc., Vol. 92-1, p. 158 ff., as well as J. A. Bondur et al., Electrochem. Soc. Proc., Vol. 90-1, p. 176 ff., use gas mixtures which comprise HBr, He/O.sub.2 or NF.sub.3. Typical etch rates (in RF plasmas) are 0.1 .mu.m/min to a maximum of 0.7 .mu.m/min and typical etch depths amount to a maximum of 20 .mu.m. However, for the manufacture of micromechanical structures, substantially higher etch rates have to be achieved. Using SF.sub.6 in combination with compact microwave- or helicon plasmas, etch rates up to 10 times greater can be realized.
Lower temperature or cryogenic etching has been shown to improve etch anisotropy (cf. W. Varthue et al., "Electron Cyclotron Resonance Plasma Etching of Photoresist at Cryogenic Temperatures", J. Appl. Phys., vol. 72, no. 7, p. 3050 (1992)). This result is thought to occur because spontaneous sidewall reactions are reduced at lower temperatures. The reduced spontaneous reaction rate reduces the etching of the sidewalls.
However, for very great etch depths, sharp anisotropic etch profiles and, simultaneously, high etch rates can not be achieved by simply etching at cryogenic temperatures. When etching to great depths, typically greater than 50 .mu.m, the side wall passivation in the area of the surface of the etched structure will, in turn, be destroyed and an isotropic profile showing a partially destroyed side wall results (1 in FIG. 1, FIG. 2).
It is therefore an object of the present invention to provide a cost-effective and reliable method for producing deep substantially vertical structures in silicon substrates.
It is a further object of the invention that these structures can be produced at high etch rates thereby showing substantially anisotropic profiles.